Wavelength Division Multiplexing (WDM) is a basic technology of networking optical signals. It is a technique by which a single fiber is used to carry many separate and independent optical channels. Each channel within the optical wavelength division multiplexed (OWDM) network is assigned a separate optical wavelength at which it is transmitted through the network. In a “sparse” OWDM system, the optical wavelengths are (relatively) widely separated. For example, two optical wavelengths of 850 nm and 1310 nm may be used in a sparse system. Such a system has an advantage of being easily implemented, but a major disadvantage is the limited number of optical channels that can be carried. In a “dense” OWDM system, the optical wavelengths are closely spaced. In a typical dense optical wavelength division multiplexed (DOWDM) system, the channel spacing may be as small as 1 nm or less. DOWDM systems provide substantially more channels than a sparse system, but are also more complex and difficult to implement.
Since OWDM networks comprise multiple communication channels, network control and routing can be achieved by directing specific channels to specific users. This concept is referred to in the art as Add-Drop Multiplexing (ADM). In ADM, an optical WDM network passes through multiple locations or nodes. At each node, one or more individual optical channels are removed (dropped) from the WDM stream and terminated in a local device. Similarly, a channel can be added into the WDM stream. Such ADM networks can be implemented using fixed wavelengths assigned to each node, or each node can be dynamically assigned wavelengths for dedicated data transmission and reception. Such dynamic ADM networks may be implemented with the wavelength allocation determined at each node or a master controller can be used to dynamically determine the wavelength allocation at each node.
OWDM networks may also provide the ability, in a given network, to allocate different services (or area of coverage) to different optical wavelengths for direct addressing. One example is in hybrid fiber coax WDM technology, where each service (broadcast video, pay per view, etc.) or different housing communities are routed by a designated wavelength in a Passive Optical Network (PON) architecture. Allocation of different services to different wavelengths simplifies the distribution of these services via optical networks, since the distribution hardware used in the networks does not need to know the type of service carried at each wavelength.
OWDM networks provide the capability to transmit large amounts of data between locations, but they have a fundamental limitation. OWDM networks require the use of optical fiber to move data from one point to another. Hence, areas where installation and maintenance of optical fiber is difficult and expensive may not be served by OWDM networks. These areas may comprise rural areas where there are few users and these users are separated by significant distances, making the provision of fiber uneconomic. These areas may also include densely populated urban areas where the costs of interfering with the infrastructure and providing OWDM fiber to multiple locations may be prohibitively expensive.
OWDM networks may still be deployed in these areas, but they will generally be deployed in a relatively small area, servicing users who are closely located. Links to users on other networks may be accomplished by tieing the networks together through the use of OWDM fiber or through the use of lower performing non-WDM data links. Coupling the separate OWDM networks through the use of OWDM fiber has the difficulty and expense factors discussed above. Non-WDM data links (such as radio frequency links or telephone lines) can be provided at a lower cost, but the networking capabilities inherent in a WDM network are lost.
Use of radio frequency links to tie together data networks is known in the art. For example, Ceragon Networks Ltd. of Tel Aviv, Israel provides products such as the FiberAir 3100 or 6200 Digital Radio System, which may be used to couple multiple Ethernet, SONET/SDH, or ATM networks. However, these systems operate at the transport level of the coupled networks, in that they recognize and process the protocol of the data transferred within the networks. This increases the overhead and complexity of the hardware used to couple the networks. In addition, these systems require network management to ensure that the coupled networks operate correctly.
Hence, there exists a need in the art for wirelessly linking separate OWDM networks with low cost, low overhead, and low complexity links while allowing transparent transfer of data between the linked OWDM networks.